Modify generator tracking to allow GENERATEs between SECTIONs

This means that code such as

```cpp
TEST_CASE() {
    SECTION("first") { SUCCEED(); }
    auto _ = GENERATE(1, 2);
    SECTION("second") { SUCCEED(); }
}
```

will run and report 3 assertions, 1 from section "first" and 2
from section "second". This also applies for greater and potentially
more confusing nesting, but fundamentally it is up to the user to
avoid overly complex and confusing nestings, just as with `SECTION`s.

The old behaviour of `GENERATE` as first thing in a `TEST_CASE`,
`GENERATE` not followed by a `SECTION`, etc etc should be unchanged.

Closes #1938

src/catch2/internal/catch_test_case_tracker.cpp

@@ -186,7 +186,8 @@ namespace TestCaseTracking {
     bool SectionTracker::isComplete() const {
         bool complete = true;
 
-        if ((m_filters.empty() || m_filters[0] == "")
+        if (m_filters.empty()
+            || m_filters[0] == ""
             || std::find(m_filters.begin(), m_filters.end(), m_trimmed_name) != m_filters.end()) {
             complete = TrackerBase::isComplete();
         }